Spectra and structures of metal-carbon monoxide compounds—II

J.Inorganic and Nuclear Chemistry,

SPECTRA

1956,Vol. 2, pp. 141-148. PergamonPress Ltd., London

AND

STRUCTURES

MONOXIDE

OF

METAL-CARBON

COMPOUNDS--II*

MANGANESE AND R H E N I U M DECACARBONYLS F. ALBERT COTTON,'~ ANDREW LIEHR, a n d GEOFFREY WILKINSON++ Mallinckrodt Laboratory, Harvard University, Cambridge, Mass.

(Received 14 September 1955) Abstract--The infra-red absorption spectra of Mn2(CO)10 and P.e~(CO)10 have been measured from 450-3000 cm -1 in the solid, in solution, and in the gas phase.

The spectra are interpreted accordingto a method of local symmetry,and two possible structures, both involving a pseudo-ring of carbon monoxide ligands lying between the metal atoms, are suggested. 1.

INTRODUCTION

IN a previous note I1) we have analysed the infra-red spectra of several cyclopentadienyl-carbon monoxide derivatives of transition elements to obtain the general features of the structure of these compounds. Using what may be called a method of local symmetry, it was shown how the number of structural possibilities could be drastically limited by observation of the number and positions of carbon-oxygen stretching frequencies. In this method, it is assumed that selected portions of a complex molecule may be treated as isolated units; that is to say, that their interactions with other parts of the molecule may be ignored. To determine the activities of the vibrations of such a subgroup of atoms, only the local symmetry of this subgroup need then be considered. It seemed possible that these principles developed for the cyclopentadienyl-carbon monoxide metal compounds could be applied to the polynuclear metal carbonyls, and in this paper we have attempted to analyse the spectra of manganese and rhenium carbonyls along these lines. Before proceeding further, it seems appropriate in connection with the method of local symmetry to note some empirical but seemingly well-established generalizations concerning the frequencies at which several types of CO groups occurring in metal-carbon monoxide compounds absorb in the infra-red. The following types of carbonyl groups may be distinguished. 1. Simple CO groups. This type of CO group exists (a) in the mononuclear metal carbonyls such as Cr(CO) 6, Ni(CO)4; (b) in the mononuclear cyclopentadienylcarbon monoxide metal compounds such as CsHsMn(CO)3, C5H5Co(CO)2; (c) ir what may be called "terminal" or "end" groups, i.e., non-bridging groups, in binuclear metal carbonyls, as, for example, in the six end-groups of iron enneacarbonyl (CO)3Fe(CO)3Fe(CO)z; (d) in complexes such as Kz[Fe(CN)sCO]. It appears certain that such CO groups bound to only one metal atom absorb at ~-2000 c m - I without an exception known to us, between 1910 cm -1 and 2060 cm 1. 2. Ketonic bridging CO groups. There is considerable evidence and reason for * Paper I in this series is reference (1). Present address: Chemistry Department, Massachusetts Institute of Technology, Cambridge, Mass. ~. Present address: Chemistry Department, Imperial College of Science and Technology, London. tl> F. A. COTTON, A. D. LIEHR, and G. WILKINSON,J. Inorg. Nucl. Chem. 1, 175 (1955). 141

142

F. ALBERT COTTON, ANDREW LIEHR, a n d GEOFFREY WILKINSON

believingtl, 2, a~ that in certain polynuclear metal carbonyls, particularly Fe~(CO)a and Cos(CO)s, there are C O groups which function as " k e t o n i c " bridges between two metal atoms, and hence absorb at frequencies characteristic of sterically similar ketone groups in organic molecules, i.e., 1700-1900 c m -1. A CO group of this type is found also in CsHsFe(CO)4Fe C5H5 .c1~ 3. Non-ketonic bridging groups. In the binuclear cyclopentadienyl-carbon monoxide c o m p o u n d s , e.g., CsHsW(CO)eWCsHs, no ketonic C O stretching frequencies were observed, and it was suggested t4~ that here bridging is affected by C O groups forming a kind of loosely-bonded pseudo-ring, f r o m which electron density was contributed to the metal atoms, with the bonding both in the ring and f r o m the ring to the metal atoms being m o r e or less delocalized. This suggestion was made to resolve the question of h o w C O groups could join two metal atoms giving a binuclear molecule, and yet not be of an essentially ketonic nature; for the C-O absorption in these c o m p o u n d s ~ is between 1890 and 1960 cm -1, overlapping the range for simple non-bridging groups. The recent w o r k by BRIMM, LYNCH, and SESNYtS) on manganese and rhenium carbonyls, Mn~(CO)lo and Re2(CO)10, showed that in spite o f the binuclear nature of the c o m p o u n d s , no ketonic bridging carbonyl absorption was observed. A closer examination of the spectra in connection with the present w o r k confirms tllis observation, and the following analysis suggests that pseudo-ring structures are reasonable for these molecules, and further, that the delocalized m o d e of bonding o f carbon monoxide groups m a y be of general occurrence. 2. E X P E R I M E N T A L We are deeply indebted to Dr. E. O. BRIMMof the Linde Air Products Co., Tonawanda, N.Y., for gifts of the metal carbonyls. Spectra were taken on a Perkin-Elmer double-beam recording infra-red spectrophotometer Model 21 in the region 3000-650 cm-1, using sodium chloride optics. The region from 650-450 cm -1 was studied using a Perkin-Elmer single-beam instrument, Model 12c, coupled with a Brown recorder, using either potassium or cesium bromide optics. Solutions were run in the usual manner, using cells with 0"4-mm spacers and fitted with appropriate halide windows. The gas-phase spectra were obtained using a cell constructed by Dr. Louxs LINDEMANof this laboratory. This hot cell consists of an ordinary gas cell of 4-cm path-length with halide windows cemented on with Glyptal clear varnish No. 1201 (General Electric Co.). This resin was then cured by heating overnight at 130-150 °. It is essential always to heat and cool the cell very slowly once the cement has been cured, to avoid cracking the halide windows. The heating jacket consisted of two sections of close-fitting brass tube wound with nichrome wire and covered with asbestos tape. These slip over the cell and are held by springs. The jacket projects about 3 cm beyond each end of the cell and is covered at each end by halide windows loosely bound to the asbestos board end-pieces of the heating jacket. The windows of the cell are in this way kept as hot as the centre portion, and condensation of material on the windows is avoided. For the vapour-phase studies the cell was maintained at I00-150°C. While quantitative vapourpressure data are lacking for these substances, the pressures were doubtless of the order of a few millimetres. 3. D A T A A N D A N A L Y S I S The infra-red absorption spectra of Mn2(CO)I o and Re2(CO)10 were studied in the range 450-3000 c m - L The spectra in the C-O stretching region, 1700-2200 cm -1, are c~ SnELINE, R. K. and PITZER, K. S., J. Amer. Chem. Soc. 71, 1107 (1950). I:~) CABLE, J. W., NYHOLM, R. S., and SHELINE, R. K., ibid. 76, 3375 (1954). ~4~ WmKINSON, G., ibid. 76, 209 (1954). ~5~ BmMM, E. O., LYNCH, M. A., and SESNY, W. J., ibid. 76, 3831 (1954).

Spectra and structures of metal-carbon m o n o x i d e c o m p o u n d g - - I I

143

J

/ij v

f I I t

tt

O. Mn 2 (CO)Io

k, i 2200

I

t

2100

2000

1900

FREQUENCY

1800

1700

IN CM "l

(a)

/" L I

II tl

/

t! t/

1i b

220~

21OO

2000

19OO

Re2(GO)lo

18OO

170q

FREQUENCY IN CM "1

(b)

FIG. 1.--Carbonyl stretching spectra in gas phase ( - - - - ) and in CHCI 3 solution (- - -). (a) Mn=(CO)z0: gas at ,~120°; solution, ,'--0.5 m g per ml CHCI3. (b) R%(CO)z0: gas at ~ 1 4 0 ° ; solution, ~ 1 mg per ml CHC13.

shown in Fig. 1, and the principal absorption bands are listed in Table 1. In both compounds there are only three frequencies, all very strong, in the neighbourhood of ~ 2 0 0 0 cm -1, and no absorption between 1700 cm -1 and 1900 cm-t. (6l 's~ The spectra of BRIMM, LYNCH, and SESNyts~ showed three strong bands in the C-O stretching region but also a weak shoulder at longer wavelengths in each case. No evidence of these shoulders was found in this study.

144

F. ALBERTCOTTON, ANDREW LIEHR, and GEOFFREYWILKINSON

In v~cwof the absence of ketonic bridging frequencies in Mna(CO)10 and Re~(CO) 1o, BRIMM, LYNCH, and SF.SNYsuggested that the binuclear nature is attributable to a metal-metal bond. The only other reasonable alternative is that, between the metal atoms, there is a pseudo-ring of carbon monoxide groups. Before applying the method of local symmetry to the problem of Mn~(CO)I o and Re2(CO)t 0, a brief r6sum6 of the assumptions of the method seems apposite. The validity of the method of local symmetry depends on the fulfilment of the following assumptions: (a) that a molecule can be resolved into relatively non-interacting vibrating groups, (b) that due to the small interaction of such groups, the selection rules of the isolated groups will be valid for the group when it is non-isolated, i.e., when it is in the molecule, irrespective of the symmetry of the whole molecule. The groups chosen in (a) are usually those of highest symmetry. This allows one to use group theory in determining the appropriate selection rules. If a molecule contains several equivalent subgroups, we assume that, due to the weakness of the vibrational interactions, the frequencies due to each of these equivalent clusters will be identical. For the various models for Mn2(CO)t0 and Re,(CO)t0 to be considered below, it is convenient to adopt a notation descriptive of the arrangement of CO groups. The most general model for any binuclear carbonyl would be represented by (x-y-z), where x and z denote the numbers of terminal simple CO groups bound to only one metal atom, and y denotes the number of CO groups placed between the metal atoms. No geometry or symmetry is implied by such a designation. For Mna(CO)t 0 and Rea(CO)t0, only models in which x : - z need be considered, since any model with x =/= z would necessarily show more than three C-O bands. The first models to be considered, which are several containing metal-metal bonds and therefore designated (5-0-5) models, serve to illustrate the notation. In Table 2 the results of applying the method of local symmetry to seven possible models are summarized. Models 1 and 2 require metal-metal bonds, whilst in the others there are pseudo-rings of 2, 4, 6, 8, and 10 carbon monoxide groups. All these models, with the exception of model 3 (vide infra) are consistent with the absence of ketonic bridging frequencies. As an example of the method of calculation used in obtaining Table 2, we shall consider the model 4 (3-4-3). The molecule is first resolved into two groups of carbon monoxide ligands each having the symmetry Caw,and one group having the symmetry D4~. Using customary group theoretical methods, we find that the C3, groups give two allowed frequencies of species At and E. Similarly, the D4~ group has two allowed frequencies of species A2~ and E,. However, for this second group, a C-O stretching mode of symmetry A2u does not exist, and thus for the whole molecule, only three distinct infra-red active frequencies would be expected. 5-0-5 models. Four possible models have been considered. The pyramidal (5-0-5) models consist of two M(CO)5 groups, each forming a pentagonal pyramid and joined at the metal atoms so as to have a common five-fold axis. Both the staggered (Ds~) and eclipsed (Ds~) configurations have been considered. In the octahedral (5-0-5) models, each metal atom is octahedrally co-ordinated by five CO groups and the other metal atom; again, staggered and eclipsed configurations are possible.

Spectra and structures of metal-carbon monoxide compounds--II

145

It is seen that all four (5-0-5) models may be eliminated, since only two C-O stretches would be expected in each case. While it might be possible to construct, ad hoc, one, or perhaps several, (5-0-5) models giving three bands, they would be of rather peculiar symmetry. We consequently consider these highly unlikely, since the metal carbonyls appear to assume as symmetrical structures as possible, as far as can be ascertained at present. TABLE 1.

SOME OBSERVED INFRA-RED FREQUENCIES

IN Mn2(CO)lo AND Re2(CO)lo Frequency, cm-1 Mn~(CO)lo

Re~(CO)~o

2060 2014 1989

Physical state of sample

Assignment

Solid in KI wafer

2076 2028 1996

2067 2008 1965

Solution in CHCI3

2061 2027 1998

2065 2006 1968

Solution in CSz

2063 2028 1997

Carbonyl stretching frequencies

Solution in CC14

2068 2039 2006

2070 2019 1985

648 642

582 574

Gas

Solution in CS2

Metal-carbon stretching frequencies

It will be shown below that by consideration of carbon-metal stretching frequencies it is possible to eliminate conclusively all (5-0-5) models. 4-2-4 model. This model agrees with experiment with respect to the number of bands, but the concept of a pseudo-ring of only two C-O groups seems to be a contradiction in terms; thus, such a model would be expected to show ketonic stretching frequencies, and is consequently discarded. 1-8-1 and 0-10-0 models. These models include only one possibility consistent with experiment, namely the D4e (1-8-1) model. The existence of such a large puckered ring seems intrinsically unlikely on chemical and intuitive grounds, and is hence discarded. 3-4-3 and 2-6-2 models. These remain the only possibilities for models consistent with the observed spectrum and not subject to any other obvious objection. We have then model 4, a (3-4-3) model with a planar four-membered ring (D4h), and

146

F. ALBERT COTTON, ANDREW LIEHR, and GEOFFREY WILKINSON TABLE 2. INFRA-RED ACTIVE CARBONYL FREQUENCIES FOR VARIOUS MODELS OF Mn~(CO)lo AND Re2(CO)lo IR-allowed ring frequencies

Total No. of active CO frequencies

--

2 for Dsa or Dsn

Structure Number

Structural type

IR-allowed end group frequencies

I

(5-0-5) pyramidal, Dsa or Dsh

A2., E~. for Dsa " E ' 1 for Dsh A2,

2

(5-0-5) octahedral

B~, E~ for D4a A2., E. for D4h

3

(4-2-4) pyramidal

A~, E for local symmetry C ~

Bs, for local symmetry D2h

3

4

(3-4-3) pyramidal

A~, E for local symmetry C3~

E , for symmetry D4h B2, E for symmetry D2a

3 4

(2-6-2)

Aa, B~ for local symmetry C2~

E~. for local symmetry D6n A2., E2u for local symmetry D3a

2 for D4a or D4h

(1-8-1)

El. for local symmetry Dsn B2, E~ for local symmetry D4a

(0-1o-0)

E'a for local symmetry Dt0h A2,, El, for local symmetry Dsd

model 5, a (2-6-2) model with a planar six-membered ring. These models are illustrated in Fig. 2. From the spectroscopic data it is not possible to decide which o f these two models is the more likely. It may be noted that in CsHsMO(CO)eMoCsH5 ~7) and in CsHsW(CO)eWCsHs, which have very similar spectra, it was found that the pseudo-rings were necessarily puckered, whilst for Mn2(CO)x o and Reg(CO)10, only planar or nearly-planar rings ~sr will fit the observed spectrum.

The Nature of the Bonding in the Pseudo-ring Carbonyl System While it is not possible to discuss the nature of the bonding in the pseudo-ring system or the bonding between the systems and the metal atoms in great detail, a few remarks seem to be well founded and apposite. (7) The existence of a compound CsHsMo(CO)sMoCsH~ seems to be dubious and the compound C~H6Mo(CO)eMoCsH5 prepared by oxidation of CsH~Mo(CO)sH has the same crystal structure as the tungsten compound. [See T. S. PIPER and G. WmKINSON, Naturwiss. 42, 625 (1955) and F. C WILSON and D. P. SHOEMAKER,Naturwiss. (in press).] ~8) If the deviations from planarity are small, the methods of reference (1) yield a resultant spectrum similar in nature to that of the planar model. See also A. D. LIEHR,J. Chem~ Phys. (in press).

Spectra a n d structures of metal-carbon m o n o x i d e c o m p o u n d s - - l l

147

The term pseudo-ring has been used with the understanding that the bonds between the carbon atoms constituting these rings of CO ligands are believed to be weak. This statement, as well as the assumption that no individual CO group in the ring forms a definite localized bond to the metal atom, follows from the fact that the absorption in the infra-red by these CO groups occurs in or only slightly below the range for simple terminal CO groups. Thus the electron density in these ring CO ligands is not greatly different from that in the terminal CO groups. It is therefore 0 m

0

0

~'C

0

CI

I-7 0

~'

0

0

~,, ,~0 O~ / C "/

_~0

C~

0

(a)

*

N 0 (b)

FIG. 2.--Acceptable models for Mnz(CO)10 and Re2(CO)10. (a) The (3-4-3) model. (b) The (2-6-2) model.

unlikely that the amount of electron density withdrawn for metal-ring bonding per CO group is very great. A further indication of this is to be found in the observation that only two absorption bands are observed in the region of carbon-metal stretching frequencies (Table 1). For either of the structures preferred for Mn2(CO)I 0 and R%(CO)I 0, these frequencies are most logically assigned to the terminal CO groups, for in each of these models the terminal groups alone require just two carbon-metal stretching frequencies. Since no strong absorption bands other than those listed in Table 1 were found, it seems reasonable to conclude that the stretching frequencies of the ring carbon-metal bonds occur below 450 cm -1, and therefore that these bonds are individually rather weak. In compounds containing ketonic CO bridges, the relatively strong localized carbon-metal bonds formed by the bridging CO groups occur in the same range as the carbon-metal stretching frequencies of the terminal CO groupsJ 91 It may be also noted here that the occurrence of only two carbon-metal stretching frequencies, although there are three carbonyl stretching frequencies, conclusively eliminates any (5-0-5) model, for in any (5-0-5) model, all carbon-metal bonds would form a set symmetrically equivalent to the carbon-oxygen bonds, tl0~ and therefore the number of carbon-metal frequencies would have to equal the number of C-O frequencies. The diamagnetism of Mn2(CO)a 0 and Rez(CO)I o may be explained on the basis of pseudo-ring structures, without the assumption of metal-metal bonds or of pairing of spins on the metal atoms. It is only necessary to assume that an odd number of electrons from the ring enter the orbitals of each metal atom. It is in fact possible to consider the primary metal to pseudo-ring bonding here as an example of the delocalized type of bond exemplified by the metal to cyclopentadienyl-ring bonds in the ~r-cyclopentadienyl compounds of transitional metals. II1~ 19~ This statement is based upon unpublished work by the authors on Fe~(CO)o, Fea(CO)12, and Co~(CO)~. ~iol This statement depends upon the fact that M-C-O grouping (for end-groups) is linear. ~11~MOFFITT, W., J. Amer. Chem. Soc. 76, 3386 (1954).

148

F. ALnlmTCOTTON,ANDREWLml-m,and GEOFFREYWILKINSON

With this type of bonding, a cup-shaped orbital of the metal atom overlaps orbitals of suitable symmetry in a ring of carbon atoms. On such a basis the (1-8-1) model would be virtually excluded, since such a ring would be too large to permit effective overlap with the metal orbital. The latter has a size suitable for good overlap with a five-membered ring, and probably with four- or six-membered rings also.

Acknowledgements--We are indebted to the General Electric Company for a Swope fellowship (F. A. C.) and to the National Science Foundation for a pre-doctoral fellowship (A. D. L.). This work was supported in part by the Atomic Energy Commission. Errata in reference (1): In eq. (2) and (5), which are kinetic energy expressions, the reduced mass # appears in the denominator, whereas it should of course be in the numerator. This error does not affect any of the end results, which were only intensity ratios.